Implement scoped_const_refptr template.

webrtc/common_video/include/video_frame_buffer.h

Index: webrtc/common_video/include/video_frame_buffer.h
diff --git a/webrtc/common_video/include/video_frame_buffer.h b/webrtc/common_video/include/video_frame_buffer.h
index cf314fe9327dce39ef9a1d8fa5d9ca14d460bedd..638ff22ca5bc85ad1b5d0f1ba86d6831d799a0e7 100644
--- a/webrtc/common_video/include/video_frame_buffer.h
+++ b/webrtc/common_video/include/video_frame_buffer.h
@@ -124,11 +124,11 @@ class I420Buffer : public VideoFrameBuffer {
 
   // Create a new buffer and copy the pixel data.
   static rtc::scoped_refptr<I420Buffer> Copy(
-      const rtc::scoped_refptr<VideoFrameBuffer>& buffer);
+      const rtc::scoped_const_refptr<VideoFrameBuffer>& buffer);
 
   // Scale the cropped area of |src| to the size of |this| buffer, and
   // write the result into |this|.
-  void CropAndScaleFrom(const rtc::scoped_refptr<VideoFrameBuffer>& src,
+  void CropAndScaleFrom(const rtc::scoped_const_refptr<VideoFrameBuffer>& src,
                         int offset_x,
                         int offset_y,
                         int crop_width,
@@ -136,14 +136,14 @@ class I420Buffer : public VideoFrameBuffer {
 
   // The common case of a center crop, when needed to adjust the
   // aspect ratio without distorting the image.
-  void CropAndScaleFrom(const rtc::scoped_refptr<VideoFrameBuffer>& src);
+  void CropAndScaleFrom(const rtc::scoped_const_refptr<VideoFrameBuffer>& src);
 
   // Scale all of |src| to the size of |this| buffer, with no cropping.
-  void ScaleFrom(const rtc::scoped_refptr<VideoFrameBuffer>& src);
+  void ScaleFrom(const rtc::scoped_const_refptr<VideoFrameBuffer>& src);
 
   // Create a new buffer with identical strides, and copy the pixel data.
   static rtc::scoped_refptr<I420Buffer> CopyKeepStride(
-      const rtc::scoped_refptr<VideoFrameBuffer>& buffer);
+      const rtc::scoped_const_refptr<VideoFrameBuffer>& buffer);
 
  protected:
   ~I420Buffer() override;